1. Field of the Invention
The invention relates to a sensor for reducing gas molecules, the sensitive element of which is tin oxide.
2. Description of the Related Art
The detection and measurement of the concentration of harmful molecules in an atmosphere constitutes a very major challenge, especially because of the increase in emissions of industrial or municipal origin. It has become necessary to detect the presence of harmful molecules in ever decreasing concentrations, below the concentrations of danger to humans. The case of carbon monoxide CO, found in particular in exhaust gases and in cigarette smoke, is one example, this gas being fatal at extremely low concentrations, of the order of 1 ppm. Furthermore, these same molecules must be detected in certain industrial or technological devices such as fuel cells, in which very low concentrations cause, for example, catalysts to be poisoned. The sensitivity threshold and the speed of detection are the two important parameters of a sensor of this type.
It is known to use tin oxide sensors to detect reducing gas molecules. The conductivity of tin oxide, which is an intrinsic semiconductor, varies according to the content of reducing molecules in the atmosphere. It is accepted that, in a nonreducing atmosphere, oxygen is adsorbed at the grain boundaries and partially repels the grain boundary electronic states, making the material highly resistive. In the presence of reducing molecules, the oxygen concentration at the grain boundaries decreases, thus releasing the electronic states near the surface of the grains, on either side of the actual contact surface. The existence of these electronic states allows conduction of electrons across the grain boundaries. It is thus apparent that the sensitivity of the sensors having tin oxide as sensitive element increases when the oxide grain size decreases, that is to say when the specific surface area increases and when the quantum effect of repelling the electronic states toward the interior of the grain is more effective. However, reducing the grain size in general causes a reduction in the physical connectivity between the grains, to the detriment of the conductivity.
Such sensors whose sensitive element is a tin oxide are described, for example in the special issue “Gas-sensing Materials” of Materials Research Society Bulletin, published in June 1998. They comprise a polycrystalline tin oxide layer on a support. In commercialized devices, tin oxide grain size is in general of the order of 1 μm. Also known are sensors containing polycrystalline tin oxide as active material in the form of grains having a size of less than 20 nm. These are essentially laboratory devices, produced by preparing the oxide from a metal obtained by sputtering, by laser ablation, by sol-gel processes, by precipitation or by other chemical synthesis processes. When the oxide is obtained by a sol-gel process or by simple precipitation, it contains a solvent that has to be removed. In most cases, the oxide obtained is in the form of a powder of separate grains, which then has to be sintered and fired so that it can be used for a sensor, and the physical connectivity of the grains is not guaranteed. Furthermore, sintering results in a three-dimensional material in the form of wafers that contain a large number of grains through the thickness. The response time of a sensor containing such an oxide will be longer the thicker the wafers, because of the time needed for the gas molecules to be detected to diffuse into the core of the active material of the sensor. Another drawback with sintering lies in the fact that it does not allow a sensor to be formed. The powders may be formed by processes consisting in incorporating the grains in an ink and then tracing the sensor using the techniques used in ink-jet printers. However, this technique gives films in which the grains are not connected on a macroscopic scale in relation to their size and it introduces solvents into the grains.